Printed wiring boards and their frames of the character described, for example, in U.S. Pat. No. 4,002,381 of D. R. Wagner et al., issued Jan. 11, 1977, are well-known in the art and have long provided an advantageous means for assembling and mounting relatively large numbers of electrical components. Typically, circuits extending from the components in the form of printed wiring on the board are extended to terminal contact areas arranged along a leading edge of the board. These terminal areas are spaced to mate with corresponding contact springs of a connector which may be mounted on the leading edge of the board. Suitable frames and racks are provided to support a number of the boards and also provide a means for guiding the boards so that their connectors may be accurately mated with terminal pins extending from a conventional backplane.
Not only must the board terminals be accurately and positively mated with the backplane pins, but the interconnection of the correct board and only the correct board with its pin field must be ensured. A power pin, for example, connected to the wrong board terminals could seriously damage the board electrical components and cause other costly damage. For this reason, a keying block is frequently interposed between the backplane and the circuit board connector. In one known backplane arrangement, such a block is fitted on terminal pins assigned thereto and has extending outwardly from the backplane, a number of keying pins uniquely positioned as a code for a particular circuit board. The connector of the latter board is provided with holes located to correspond to the unique positioning of the keying pins. As a result, only the correct circuit board connector can be fitted to the keying block pins and, therefore, to the correct backplane terminal pin field. All other circuit boards with connectors not having holes of the unique locations will be prevented from receiving the keying pins and, therefore, the wrong terminal pin field.
After its installation on a backplane assembly, it may become necessary on occasion to remove a keying block from its supporting terminal pins. The block may, for example, have been erroneously fitted, or it may have become damaged during the fitting of a circuit board. Before the installation of the circuit boards in their guide frames, a keying block may, if need be, be removed by hand. The very close spacing of adjacent boards in a fully assembled system, however, leaves insufficient room for such manual removal or replacement of a keying block. Manual access to a keying block would also not be recommended in view of the risk of inadvertently making contact with a powered backplane pin. It is thus the problem of gaining access to, removing, and replacing a keying block of a backplane to which the apparatus of the invention is chiefly directed.